Dénes, J.

Keedwell, A. D.

English

Periodica Polytechnica (Budapest University of Technology and Economics)

ON TWO CONJECTURES RELATED TO ADMISSIBLE GROUPS AND QUASIGROUPS To the memory of Prof. K. Buzasi J. DEems and A. D. KEEDWELL* 1122 Budapest, Hungary *Dept. of Mathematical and Computing Sciences Dniv. Surrey, Guildford, England Received August 1 L 1988 A finite group (quasigroup) is said to be admissible if it has a permuta-tion mapping of the form g -+ IX( g) such that g -+ glX( g) is also a permutation. The study of admissible groups is an important subject. To mention just one of the applications of admissible groups, we call the reader's attention to the fact that if L denotes the latin square which represents the multiplication table of a group of odd order, then L has an orthogonal mate (see Theorems 1.4.3, 5.1.1 in [2]). More than thirty years ago L. J. P AIGE and lVI. HALL proposed two conjectures for non-soluble groups: (1) if the product of all elements, in some order, is equal to the identity element, then the group is admissible; (2) if the Sylow 2-groups are non-cyclic, then the group is admissible (see [9], [4] and Problems 1.5, 1.6 in [2]). In [3] it has been proved that the two conjectures above can be replaced by the following one: All non-soluble finite groups are admissible. The aim of the present note is to formulate a new conjecture and to prove that the above-mentioned conjecture is implied by the new one. A square array of size n hy n with n2 elements in 'which each row contains each distinct element once is called a row-Iatin square (Similarly, a column-latin square is a square where each column contains each distinct element once). A square which is both column-Iatin and row-Iatin is a latin square. (For a detailed description of properties of latin squares and their generaliza-tions see [2]). In [7] and [8], a product operation for latin squares has been introduced. Let the distinct elements of an n X 11 row-Iatin square be designated by the integers 0, 1, ... , n - 1. Then the i-th row of the square determines a permutation lXi of these integers from their natural ordering. The square is completely determined hy the permutations 1X1 = C, 1X2' 1X3' ••• , IX" which define 3 34 J. DElVES and A. D. KEEDWELL its rows as permutations of the first ro,,-. The product of two row-Iatin squares A = ((ZlX2 ••• (Znf and B = (P1P2 ... Pnf is defined to be the row-Iatin square AB = (X1P1' (Z2/32 ••• (ZnPnf whose i-th row is given by the product permutation (Z;/3;. Clearly, a consequence of this definition is that Y A = (V (Zl V (Z2 ••• Y (Zn) and that this row-latin square exists if and only if each of the permutations Xi' i = 1,2, ... , n, has a square root. Now, the square of a cycle of odd length is another cycle of the same odd length, the square of a cycle of singly-even length 4t + 2 is a product of two cycles of odd length 2t + 1 and the square of a cycle of doubly-even length 4t is a product of two cycles of even length 2t. Thus, the square of any permu-tation has an even number of cycles of even length. Moreover, any permuta-tion with the latter property has at least one square root since and ((11(12' •• a2t )(b1b2 ••• b2t ) = ((11 bl(1zb z •.. (12tbzt)2. If the permutation has more than one cycle of equal odd length then it has at least two (and possibly many) square roots since each product of two cycles of equal odd length has two distinct square roots. For example, (0 5 1 6 2 7 3 8 4 9) and (0 3 1 4 2)(5 8 6 9 7) are both square roots of the cycle product (0 1 2 3 4)(5 6 7 8 9). For fmther results on square or higher roots of permutations, see [1], [5], [6]. Next, 'we remak that, if (G, . ) is a finite non-soluble group, then its multiplication table is a latin square L such that exists. Proof. Let L (x1xZ ••• xnf. Then the permutation Xi is the Cayley representation of the i-th element gi of G and so it is a regular permutation with cycles of length equal to the order of the elemf)nt gi in G. Let us suppose that, for at least one value of i, Xi does not exist. In that case, Xi consists of an odd numher k of cycles of even length, where k divides ord G. Let ord G = n = 2Thk, "where hand k are odd integers and r ;;::: 1. Then each of the cycles of (Zi has length 2Th and so x~' has order 2r. That is, the element g;' of G which is repre-sented by rx'l generates a cyclic Sylow 2-subgroup of G. However, it is well-known (see, for example, Theorem 2.10 of [10]) that the Sylow 2-subgroups of a non-soluble group are not cyclic. This contradiction shows that ~ must exist. Comhining this with our previous conjecture in [3] that all finite non-soluble groups are admissible, 'we are led to make the following further con-jecture: Conjectllre 1. If L is the multiplication table of a non -soluble group then, not only does L have at least one, and possibly many, square roots VL, but at least one of these square-root squares is a latin square. CONJECTURES RELATED TO ADMISSIBLE GROUPS 35 If the conjecture were true, it would follow that the latin squares Land VE arc orthogonal and consequently that G is an admissible group. To see this, we remark that (1"£)-lL = rE and so the result follows from a theorem of H. B. MANN [7]. We end this note by comhining Mann's result with our conjecture above to give: Conjecture 2. A necessary and sufficient condition for a latin square A to have an orthogonal mate is that either A2 is a latin square or A can be repre-sented as the product A BC, of two not-necessarily-distinct latin squares Band C. The sufficiency is clear: only the necessity remains in doubt. References 1. C:-CPOiSA. G., SA:'I!ARDZISKI. A., CELAKOSKL N.: Roots of permutations. Proceedings of the Symposium n-ary structures, Skopje 1982, 85-95. 2. DiNES. J .• KEEDWELL A. D.: Latin squares and their applications, Academic Press, l\"ew York, A.kademiai Kiad6. Budapest, English Universities Press, London, 1974. 3. DiNES. J., KEEDWELL, A. D.: A new conjecture concerning admissibility of groups, European Journal of Combinatorics. 18 (1989) 171-174. 4. HALL, M .• P . .\.IGE, L. J.: Complete mappings of finite groups, Pacific J. "Math. 5 (1955), 541-549. 5. HIGGmS, P.2\1.: A method for constructing square roots in finite full transformation semi group, Bull. Canadian Math. Soc. 29 (1968), 344-35l. 6. HOWIE, I. "1., SNOWDEN, M.: Square roots in finite full transformation semigroups, Glasgow Math. J. 23 (1982), 137 -149. 7. ::\-L\.NN, H. B.: The construction of orthogonallatin squares, Ann. ::\fath. Statist., 13 (1942) 418-423. 8. NORTONS, D. A.: Groups of orthogonal row-Iatin squares, Pacific J. l\1ath. 2 (1952), 335-431. 9. PAIGE, L. J.: A note on finite abelian groups, Bull. Amer. 2\1ath. Soc. 53 (1947), 590-593. 10. SUztTKL:c\1.: Group theory n. Springer Verlag. New York, 1968 J. DiNES 1122 Budapest, Csaba u. 10. Hungary A. D. KEED\YELL Dept. of Mathematical and Computing Sciences Dniv. Surrey, Guildford, England 3* 

ON TWO CONJECTURES RELATED TO ADMISSIBLE GROUPS AND QUASIGROUPS

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ON TWO CONJECTURES RELATED TO ADMISSIBLE GROUPS AND QUASIGROUPS

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